Serum Magnesium Levels as a Prognostic Marker in Emergency Department Admissions: A Landmark Retrospective Cohort Study.

Objectives: Early identification of serious hypermagnesemia in emergency department (ED) patients is challenging due to limited clinical information and unknown baseline factors. Our institution implemented routine serum magnesium (Mg) testing for all ED patients to prevent missed diagnoses of fatal hypermagnesemia. We evaluated whether these measurements improved short-term prognostic assessment by associating with 28-day all-cause mortality. Methods: We conducted a retrospective cohort study of 43,100 adult ED admissions at a tertiary center in Japan from January 2017 to December 2019; 12,580 met the eligibility criteria. Patients were stratified according to Mg levels: hypomagnesemia (<1.6 mg/dL; n = 433), normomagnesemia (1.6-2.4 mg/dL; n = 10,722), and hypermagnesemia (≥2.5 mg/dL; n = 1425). The primary outcome was 28-day all-cause mortality. Multivariable analyses were adjusted for age, sex, admission diagnosis, admission to the intensive care unit, albumin, C-reactive protein (CRP), and other laboratory covariates. Restricted cubic spline analysis was used to assess nonlinear associations. Results: The model adjusted for patients' characteristics showed higher mortality rates in hypomagnesemia and hypermagnesemia. Spline analysis initially revealed a U-shaped relationship between Mg levels and 28-day mortality. However, the risk associated with hypomagnesemia was attenuated after additional adjustments for CRP and albumin levels, whereas hypermagnesemia remained independently associated with 28-day mortality (adjusted hazard ratio, 1.73; 95% CI 1.51-2.00). This association remained consistent after stratification by renal function or admission diagnosis. Conclusion: Systematic Mg testing in the ED may facilitate the early identification of patients at imminent risk. Routine Mg assessment could help clinicians recognize high-risk patients in acute care.